Season of Loss

No player can cast spells or activate abilities in between the modes of a resolving spell.

You choose the modes as you cast the spell. Once modes are chosen, they can't be changed.

If all targets for the chosen modes become illegal before the Season resolves, the spell won't resolve and none of its effects will happen. If at least one target is still legal, the spell will resolve but will have no effect on any illegal targets.

When you're following the instructions from the first mode, first the player whose turn it is chooses which creature they're going to sacrifice, then each other player in turn order does the same. Players will know the choices of previous players when making their choices. Then all chosen creatures are sacrificed by their controllers simultaneously. Repeat this process for each additional time the first mode was chosen.

The pawprint symbol does not represent a cost, mana, counters, or any kind of persistent resource. You can't "save up" pawprint symbols from one Season spell to use on a future one, mostly because there isn't anything concrete to save up. They're just pawprints.